Recently, a motor with a small cogging torque is required in diverse applications. Examples include industrial-level servo motors and elevator winches. When attention is focused on such applications for vehicle, an electrical power steering apparatus is in widespread use for the purpose of achieving better fuel consumption and better steering performance. In the case of a motor used in the electrical power steering apparatus, because a cogging torque is transmitted to a driver via a gear, there is a strong demand for a cogging torque reduction of the motor in order to obtain a smooth steering feeling. To satisfy such a demand, there is a method of providing supplemental grooves in an iron core of a stator as a method of reducing a cogging torque as described, for example, in Patent Documents 1, 2, and 3.